Graphite and carbon fiber reinforced resin matrix composites are finding increased use in many military and industrial applications. The significant benefit of resin matrix composites (or laminates) is weight savings in structures and systems over those fabricated from reinforced or non-reinforced metallic and ceramic materials without sacrifice of mechanical strengths. The weight savings equate directly to increased energy efficiency and payload capabilities. Composites are used today in many primary airframe, missile, spacecraft and vehicular structures, to cite a few applications.
A desire in the evolution and development of advanced fiber reinforced composites has been to apply these materials in increasingly hostile thermal environments. A secondary goal has been to increase the inherent impact strength of composite materials without sacrificing their other desirable characteristics and properties. In general, increased thermal stability is gained by incorporating a significant aromatic and heterocyclic content into the matrix resin structure and increased impact strength is gained by using thermoplastic matrix resins instead of thermosetting materials.
Significant improvements in the thermal stability of thermosetting composite matrix resin materials have been described by NASA in U.S. Pat. No. 3,745,149 for norbornene terminated polyimides and by Hughes Aircraft Company in U.S. Pat. No. 4,100,138 for ethynyl terminated polyimides. These thermosetting polyimide matrix resins are now used in primary structural applications where thousands of hours mechanical strength integrity retention at 500.degree. F. to 600.degree. F. in air is required. However, because these matrix resins are thermosetting when fully cured, their impact strengths, in general, and ability to resist severe microcracking on thermal cycling, in particular, are suspect.
In our U.S. Pat. Nos. 4,477,648 and 4,521,623 we have pointed out that a linear condensation polyimide based upon a reaction of four-ring aromatic diamine, 2,2-bis[(4-aminophenoxy)phenyl]hexafluoropropane, and pyromellitic dianhydride, as described in U.S. Pat. No. 4,111,906 to one of us, possessed promise as a matrix resin for use in jet compressor stage stator bushings at 675.degree. F. use temperature. However, the critical temperature increase from 675.degree. F. to 700.degree. F. required for the new generation of advanced aircraft engines deleteriously affects the performance of this polyimide. The only other known resin which was thought to have equal or higher promise at these high temperatures was a resin once marketed by duPont as NR-150B. This was a linear condensation polyimide based upon single ring aromatic diamines and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride as described in U.S. Pat. No. 3,356,648. This resin, however, is no longer commercially available.
The approach taken in our U.S. Pat. Nos. 4,477,648 and 4,521,623 in our quest for polymer systems suitable for use at 700.degree. F. (644.degree. K.) in air at pressures up to 10 atmospheres involved provision of a polyimide made from 2,2-bis[(2-halo-4-aminophenoxy)phenyl]hexafluoropropane where the attached ortho halogen is preferably chlorine. A polyimide made from 2,2-bis[(2-chloro-4-aminophenoxy)phenyl]hexafluoropropane and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride was found to have exceptional high temperature performance. Unfortunately, this polymer is quite expensive as it requires use of a pair of relatively expensive monomers for its synthesis.